As is generally known, control moment gyros (CMGs) are used to control the attitude of a spacecraft, such as a satellite. Presently known CMGs typically include a spin motor, a torque motor, and a momentum wheel (or flywheel). The spin motor rotates the wheel about its centerline axis at some velocity, and the torque motor rotates the wheel through a limited angle about an orthogonal axis. These two motions generate a gyroscopic torque, about a third orthogonal axis, that is proportional to the spin velocity and the tilt angle of the wheel. Typically, the spin motor is relatively small, because only a small torque is required to maintain the spin velocity (once it has reached this velocity), and because the torque motor must move the spin motor along with the wheel.
A fundamental limitation of presently known CMGs is the existence of “singularities” within the momentum envelope of the CMG system. These are specific positions at which the CMG system cannot generate any momentum, due to the alignment of momentum vectors. As may be appreciated, these positions are avoided to prevent the satellite from becoming effectively stuck in a position. One way to mitigate this problem is to mount the CMG in a two-axis gimbal and add a second torque motor. This second motor can then adjust the amplitude of the momentum vector to avoid the singularities. However, this obviously adds undesirable size, weight, and complexity to the system, especially since the second torque motor must work against the gyroscopic torque generated by the first torque motor. Another method is to vary the spin velocity of the wheel while it is being tilted, resulting in what is called a “variable-speed CMG.” In practice, however, this is rarely implemented because it requires much higher torques on the spin axis, which results in a larger spin motor, larger toque motor, and larger size and weight overall.
Hence, there is a need for a multi-degree of freedom electromechanical machine that can, for example, implement the functions of a CMG without undue size, weight, and complexity. The present invention addresses at least these needs.